A major technical challenge to the use of interstitial hyperthermia in malignant brain tumors is the production of a well-defined, uniform hyperthermal field. In theory, a 915-MHz microwave antenna should allow fewer antennas to be used and cause less mechanical brain damage; however, standard radiation afterloading catheters require antennas to be 12 cm long; this is clearly impractical for intracranial use. Since alumina ceramic (Al2O3) catheters permit short microwave antennas (3-5 cm in length) to function properly in neural tissue, it is important to test the biocompatibility of alumina for use in combined interstitial microwave hyperthermia and brachytherapy. A 5-mm length of alumina catheter was implanted into the brains of 15 white rats. The animals were killed at 3, 7, 14, 28, and 56 days. Histological examination revealed only minor mechanical damage and no encapsulation until 1 month; even then, the glial wall was only a few cell layers thick. Five animals received implants and were killed at similar intervals for x-ray microanalysis with the scanning electron microscope. No migration of aluminum into the brain was detected when compared with two control animals that did not receive implants and an alumina blank. Although we measured 50% attenuation of the radiation from iridium-192 sources in alumina catheters as compared with conventional ones, alumina catheters can still be used for interstitial radiation by increasing either the activity of the seeds or the duration of treatment.